1. Field of Invention
The present invention relates to a driving circuit. More particularly, the present invention relates to a light source driving circuit for driving a light-emitting element.
2. Description of Related Art
Light source driving circuits have been widely applied in various display devices, for example, backlight driving circuits of LCDs, or LED driving circuits of handheld mobile devices. As display devices are increasingly used in various electronic products, the light source driving circuits have a great potential for development, and many advantages of the circuits, such as power conversion efficiency and stability, can be improved and enhanced continuously.
The architecture of a conventional light source driving circuit for driving an LED string 101 is shown in FIG. 1. The LED string 101 includes a plurality of LEDs 102 strung together. The conventional light source driving circuit includes a DC/DC power converter 103, a capacitor 104, an output voltage feedback circuit 105, a timing control circuit 106, and a dimming circuit 107. The dimming circuit 107 is connected between a cathode of the LED string 101 and a common potential GND, and receives a pulse width signal PWM from the output of the timing control circuit 106. In addition, the output voltage feedback circuit 105 is implemented by two resistors 108 and 109 connected in series.
The DC/DC power converter 103 supplies a DC source voltage to the LED string 101, and performs a feedback control according to a feedback signal FB received by the output voltage feedback voltage 105, so as to stably output a DC source voltage. The timing control circuit 106 determines the width of the signal according to the luminance set by the user. The dimming circuit 107 is turned on as the signal is at a high level, and is turned off as the pulse width is at a low level. By adjusting the width of the pulse width signal, the ON/OFF state of the dimming circuit 107 can be controlled, so as to further control the LED string 101 to whether or not allow a current passing through.
Thus, when the luminance is set to high, the time that the pulse width signal remains at the high level is long, such that the duration of turning on the dimming circuit 107 is long, and the luminance of the LED string 101 is high. On the contrary, when the luminance is set to low, the time that the pulse width signal remains at the high level is short, such that the turn-on time of the dimming circuit 107 is short, and the luminance of the LED string 101 is low.
However, when the pulse width signal is at the low level, the dimming circuit 107 is turned off (i.e., the electrical path between the cathode of the LED string 101 and the common potential GND is not turned on), and at this time, the voltage drop of the DC source voltage supplied by the DC/DC power converter 103 completely falls on the dimming circuit 107. When the LED string 101 includes lots of LEDs 102 connected in series, the DC source voltage supplied by the DC/DC power converter 103 will have a high voltage value.
Under this circumstance, if the voltage tolerance of the dimming circuit 107 is not high enough, the dimming circuit 107 will be damaged by the DC source voltage supplied by the DC/DC power converter 103. Moreover, the dimming circuit 107 with a higher voltage tolerance can be used. However, the dimming circuit 107 with the higher voltage tolerance requires larger space and has a higher price, which not only increases the volume of the entire circuit, but also increases the cost. Thus, the market competition force of this product is reduced.